Communication, sensing, and imaging devices today are moving towards higher operation frequencies and broader bandwidths to enable higher communication speeds. A consequence of higher operation frequencies is that the size of the components in the devices must be decreased. This is an important driving force for miniaturization of devices and the components that constitute them. Further advantages of miniaturization are that the devices can become lighter and cheaper, with lower power consumption and with a larger number of functions integrated into the same volume.
Waveguides are used to transmit electromagnetic wave signals from one position to another. When using waveguides, it is desirable to transmit the full guided power from one point to another, with no or only minor effect losses. To achieve this, waveguides are often closed, having a pipe-like construction. If the pipe-like waveguides are hollow with metallic walls, the wave is efficiently confined. However, such structures are rigid, which is not always desirable. Furthermore, metallic hollow structures at very high frequencies are difficult to fabricate as miniaturized waveguides where high precision 3D manufacturing process is required. Hence, alternative types of micromachined waveguides have been developed to overcome these restrictions [1]. However, all of these structures necessitate costly manufacturing techniques.
A new concept for waveguides called substrate integrated waveguides (SIWs) has therefore been introduced [2-4]. SIWs can be seen as rectangular pipe-like waveguides that are incorporated into the substrate itself. They generally consist of two vertical walls, a top and a bottom layer, with a dielectric material, i.e. the substrate itself, in between. In this way, SIWs can also be seen as pipe-like, but with the additional merits of miniaturization, low cost and ease of integration with other components on the same substrate. Several SIWs have been demonstrated for use in a number of different components for devices at frequencies ranging from around 10 GHz to 180 GHz [5-8]. Demonstrated SIWs that can operate at frequencies above the Ka-band (26.5-40 GHz, often denoted microwave band) are fabricated using relatively costly fabrication techniques and materials.
SIWs are traditionally manufactured using rigid printed circuit board (PCB) fabrication techniques to reduce fabrication costs. In this case, the vertical walls of the SIWs are not continuous metallic walls, but rather rows of metallized through-hole vias or posts, see FIG. 5A. The size and spacing of these metallized vias are in the order of several hundred micrometers. The through-holes in the circuit board are formed by mechanical drilling or by laser techniques. The prior art technique results in significant gaps between the vias of several hundred micrometers, causing some of the guided power to radiate out of the waveguide and is therefore lost. Correspondingly, power from adjacent power radiating structures, such as nearby antennas, can radiate into the waveguide through the gaps, thereby contributing to noise and interference.
A PCB SIW with metallized vias having a diameter of 0.3 mm and a vias spacing of 0.6 mm has been fabricated [16].
The leakage loss is frequency dependent, and the higher the frequency the higher the losses. Consequently, SIWs fabricated by printed circuit board fabrication techniques and having large vias diameters and spacings cannot operate at high frequencies. Previously presented SIWs fabricated with printed circuit board fabrication techniques have been demonstrated at frequencies up to Ka-band [9-10].